1. Field of the Invention
The invention relates to a process for preparing a random monoalkenyl arene-conjugated diene copolymer. More particularly, the invention relates to a process for preparing an anionic random monoalkenyl arene-conjugated diene copolymer employing a photometer to control the rate of addition of the conjugated diene monomer.
2. Description of the Prior Art
The copolymerization of conjugated dienes and styrene has been widely utilized for some time. The most commonly used process for the copolymerization has been by an emulsion technique utilizing a free radical catalyst such as an organic peroxide or hydroperoxide. More recently, styrene-diene random copolymers have been prepared by the solution polymerization of styrene and a conjugated diene with organolithium initiators.
With regard to such lithium initiation, it is known that in the batchwise copolymerization, the diene monomer polymerizes considerably faster than the styrene monomer. As a result of this, if no special measures are taken, tapered block copolymers are formed (see e.g. U.K. Pat. No. 888,624).
Several attempts have already been made to counteract this block formation and to promote the formation of copolymers of which the monomer units are distributed more statistically over the polymer molecule. For instance, a process has been proposed in which the two monomers are added at a rate lower than the normal polymerization rate of the system under the conditions applied (U.S. Pat. No. 3,094,512), which means that the monomers are added in such a way that a complete reaction takes place while the addition is going on. This implies that, if application of a relatively low polymerization temperature, for instance, below 110.degree. C., is desired, the monomers should be added very slowly and in accurately determined quantities, as a result of which the process is time-consuming and the polymer yield per unit of time small.
By polymerization at higher temperatures the monomer addition can indeed be made to proceed faster, but then thermal decomposition of the initiator may take place at an unacceptable rate. In addition the harmful influence of the higher temperature on the polymer formed is greater. Moreover, when styrene is applied as the vinylaromatic compound, a high polymerization temperature is undesirable in view of the risk of thermal homopolymerization of the monomer by free radicals. These objections and risks carry weight at temperatures higher than 90.degree. C. and can particularly become inconvenient at temperatures higher than 150.degree. C.
In the last-mentioned process invariably only one preselected monomer ratio is employed, namely, that at which the monomers are added. The monomer ratio at which the copolymerization starts is equal to this ratio.
Another process for the preparation of statistical copolymers is disclosed in U.K. Pat. No. 994,726. In that patent, random copolymers are prepared by first forming a mixture of a portion of the butadiene monomer and all of the styrene monomer in a ratio selected to give the desired copolymer content. Then the mixture is contacted with a lithium-based catalyst under polymerization conditions. Incremental additions of butadiene monomer are added to maintain the monomer ratio. Again, in this process only one preselected monomer ratio is applied. As during the copolymerization, since no vinylaromatic compound is added, per unit of time a decreasing amount of diene should be supplied. As a result not only the concentration, but also the absolute quantity of each of the monomers present in the reactor steadily decreases. Consequently, though the monomer ratio is kept constant, the monomer concentration, which as a rule is fairly high initially, decreases to a value which is rather low at the end of the copolymerization. It is not easy to control this process since the butadiene is added in discrete, incremental amounts pursuant to pre-selected addition rates. If the calculations are wrong or if conditions change in the reactor, then the resulting polymer will not be statistically random.
Still another copolymerization process is disclosed in U.K. Pat. No. 1,283,327. In that patent both monomers are added continuously to the reactor. The rate of addition of monomer is pre-set at the rate required to maintain the specific concentration of monomers. To maintain a constant copolymer ratio (the definition of a random copolymer) the monomer ratio in the reactor must be constant and will be completely different from the ratio in the polymer. The flow of monomer to the reactor must equal the disappearance of monomer by polymerization. The resulting polymer will become non-random or tapered if the precalculated monomer supply rate is incorrect, if the initial monomer concentration is incorrect, or if the initial monomer ratio is incorrect. For example, if the desired ratio of styrene to isoprene in a polymer is 11:1, preparation of such a polymer by the process of U.K. Pat. No. 1,283,327 would require a monomer ratio of styrene to isoprene of greater than 140:1 due to the extremely high reactivity of isoprene monomer in this system. Precisely maintaining such a monomer ratio is impractical. Another consideration is the extent of thermal termination or dieout of the active polymer chains.
Another approach to making random styrene-diene copolymers is to add a randomizing agent to solution mixtures of diene, styrene and organolithium initiators. Various randomizing agents are disclosed in U.S. Pat. Nos. 2,975,160; 3,366,611; 3,496,154; 3,498,960 and 3,673,166. These strong randomizing agents are typically employed at fairly high levels of addition in order to equalize the diene monomer and styrene monomer reaction rate constants. However, this technique is not always acceptable because altering the polarity of the solvent to the extent of equalizing the rate constants would necessarily result in an unacceptably large change in the microstructure of the diene polymer units. For example, a significant increase in the 3,4 content of isoprene polymer units might result.
A new polymerization process has now been found that overcomes many of the difficulties found in the prior art.